Well shucks. Now it isn’t happening. The only difference is that I did the recent test with a copy of the image. Could there have been a fragmentation issue? In any case, here are the steps:
Open password protected sparsebundle
Open the EF library therein of a couple thousand images, variously tagged with 20 tags.
Using the tag list at the left of the browser, select a common tag, say “Landscapes.”
Control click and make a new tag from source “Best Landscapes.”
Open the tag window.
Select all images in this new collection and uncheck “Best Landscapes.”
Now clicking back on the “Landscapes” tag on the left, go through the Landscapes images and selectively tag some “Best Landscapes” by checking that tag in the tag window. While you have the image open, adjust some other tags. Do about 20.
Bring mail to the front. Read a message. (Quickeys and Spellcatcher are involved.)
Go back to EF. All the images in the tag group have been deleted. Click Command-Z to get them back.
Now click “records” in the top left of the browser. Notice that a dozen or so images — not involved in “landscapes” have had all tags deleted.